Apparatus and method for treating organic waste water

ABSTRACT

The present invention overcomes mutually contradictory problems associated with a system for organic wastewater treatment through denitrification and nitrification processes, to the effect that when the circulating volume from the nitrification tank to the denitrification tank is increased to achieve high removal of nitrogen, the introduction of a nitrified fluid rich in dissolved oxygen causes a reduction in denitrification performance of the denitrification tank. Thus, the object of the present invention is to provide a system and method for organic wastewater treatment through denitrification and nitrification processes, which system and method achieve high removal of nitrogen without reducing the denitrification performance of denitrification tanks. According to one embodiment of the present invention, the object stated above is achieved by providing a system for treating organic wastewater, which comprises a multistage treatment tank having at least two stages in series, each stage comprising a denitrification tank and a nitrification tank connected in this order, wherein said system comprises a pipe for distributing feed water to the denitrification tank in each stage, a means for filtering and separating at least a part of an activated sludge mixed liquor present in at least one nitrification tank, and a pipe for supplying at least a part of the concentrated sludge mixed liquor obtained by said filtration and separation treatment to the denitrification tank.

TECHNICAL FIELD

[0001] The present invention relates to wastewater treatment. Morespecifically, it relates to a system and method for treating organicwastewater, including organic industrial wastewater and domesticwastewater, by means of an activated sludge process to denitrify andnitrify the wastewater.

BACKGROUND ART

[0002] In conventional water treatment using activated sludge,solid-liquid separation of the activated sludge mixed liquor has beenrequired to obtain clean treated water. To this end, it is common to usea strategy in which an activated sludge mixed liquor is introduced intoa sedimentation tank to thereby deposit the sludge by the action ofgravity, while the supernatant is discharged as treated water from thesedimentation tank. In this case, the sedimentation tank is required tohave a settling area large enough and a retention time long enough todeposit activated sludge, thus causing increases in the size and settingvolume of the entire treatment system. Moreover, in a case whereactivated sludge is difficult to deposit by reason of bulking or othercauses, the sludge overflows from the sedimentation tank, which leads toreduced quality of the treated water.

[0003] In biological nitrogen removal through denitrification andnitrification processes, the nitrified fluid is circulated fromnitrification tank to denitrification tank, whereby NO_(x)—N (nitratenitrogen and nitrite nitrogen) in the nitrified fluid is reduced to anitrogen gas by the action of denitrifying bacteria present in activatedsludge. During this step, a BOD source in raw feed water is used as ahydrogen donor for denitrification. In such adenitrification/nitrification process, the circulating volume from thenitrification tank to the denitrification tank should be increased toachieve high removal of nitrogen. However, when the circulating volumefrom the nitrification tank is increased for sludge rich in dissolvedoxygen, a further problem arises because the introduction of dissolvedoxygen causes a reduction in denitrification performance of thedenitrification tank to be used under anaerobic conditions.

[0004] On the other hand, a strategy using membrane separation insteadof a sedimentation tank is conventionally used to conduct solid-liquidseparation of activated sludge. In this case, it is common to use amicrofiltration membrane or an ultrafiltration membrane as a membranefor solid-liquid separation. However, this strategy requires asuction/pressure pump as a filtration and separation means. Sincefiltration is usually accomplished under a pressure ranging from severaltens of kPa to several hundreds of kPa, a high pump power is requiredand leads to an increase in running costs. Membrane separation enablesthe provision of clear treated water completely free from SS, butpermeation flux is low, and it requires periodic chemical washing of themembrane in order to prevent membrane pollution.

[0005] In recent years, as an alternative to using a sedimentation tank,another strategy for solid-liquid separation of an activated sludgemixed liquor has been proposed in which a filter material composed of awater-permeable sheet such as woven or nonwoven fabrics, or metallic netmaterials is immersed in an aeration tank to form a secondary depositionlayer of sludge particles per se on the surface of the filter, which isthen used as a filtration layer to obtain clear filtered water at lowhydraulic head pressure. This strategy is called “dynamic filtration.”Such a filter material composed of a water-permeable sheet allows sludgeparticles to pass through itself, while a cross-flow of the activatedsludge mixed liquor generated on the filter surface causes the formationof a secondary deposition layer of sludge flocs on the water-permeablesheet. This sludge layer serves as a filtration layer (i.e., a dynamicfiltration layer) so as to achieve solid-liquid separation of sludge andSS contained in feed water. Since the thickness of such a dynamicfiltration layer increases over the course of filtration time, thethickened layer results in increased filtration resistance and hencereduced filtration flux. In such a case, aeration is performed throughan air-diffusing pipe provided below the filter to remove the dynamicfiltration layer of sludge formed on the filter surface, followed byformation of a new dynamic filtration layer to ensure stable filtrationflux.

[0006] In applying this dynamic filtration technique to thedenitrification/nitrification process, there are two possibleembodiments, one of which employs a dynamic filter module placed in anitrification tank, and the other of which employs a dynamic filtermodule placed in a solid-liquid separation tank provided separately fromdenitrification and nitrification tanks. However, in both embodiments,the circulating volume from the nitrification tank to thedenitrification tank should be increased to achieve high removal ofnitrogen; and hence, as in the case of conventional treatment, a furtherproblem arises because the introduction of a nitrified fluid rich indissolved oxygen causes a reduction in denitrification performance ofthe denitrification tank.

[0007] The present invention overcomes mutually contradictory problemsassociated with a system for organic wastewater treatment throughdenitrification and nitrification processes, as stated above, to theeffect that when the circulating volume from the nitrification tank tothe denitrification tank is increased to achieve high removal ofnitrogen, the introduction of a nitrified fluid rich in dissolved oxygencauses a reduction in denitrification performance of the denitrificationtank. Thus, the object of the present invention is to provide a systemand method for organic wastewater treatment through denitrification andnitrification processes, which system and method achieve high removal ofnitrogen without reducing the denitrification performance ofdenitrification tanks.

DISCLOSURE OF THE INVENTION

[0008] The present invention provides the following means to overcomethe above problems.

[0009] 1. A system for treating organic wastewater, which comprises amultistage treatment tank having at least two stages in series, eachstage comprising a denitrification tank and a nitrification tankconnected in this order,

[0010] wherein said system comprises a pipe for distributing feed waterto the denitrification tank in each stage, a means for filtering andseparating at least a part of an activated sludge mixed liquor in atleast one nitrification tank, and a pipe for supplying at least a partof the concentrated sludge mixed liquor obtained by said filtration andseparation treatment to the denitrification tank.

[0011] 2. The system for treating organic wastewater according to 1above, wherein the filtration and separation means is placed in thenitrification tank, and at least a part of the concentrated sludge mixedliquor obtained by filtration and separation treatment in thenitrification tank is supplied to the denitrification tank(s).

[0012] 3. The system for treating organic wastewater according to 1above, which further comprises a solid-liquid separation tank,

[0013] wherein the filtration and separation means is placed in thesolid-liquid separation tank, and at least a part of the concentratedsludge mixed liquor obtained by filtration and separation treatment inthe solid-liquid separation tank is supplied to-the denitrificationtank.

[0014] 4. The system for treating organic wastewater according to anyone of 1 to 3 above, wherein the filtration and separation means is adynamic filter comprising a water-permeable filtration layer supportmaterial for forming a dynamic filtration layer of activated sludgeparticles thereon.

[0015] 5. The system for treating organic wastewater according to 4above, wherein the water-permeable filtration layer support material iscomposed of at least one member selected from the group consisting of awoven fabric material, a nonwoven fabric material and a metallic netmaterial.

[0016] 6. The system for treating organic wastewater according to anyone of 1 to 5 above, which further comprises a sedimentation tank to beused for solid-liquid separation of at least a part of the activatedsludge mixed liquor introduced from the nitrification tank in the finalstage, as well as a pipe for returning at least a part of the depositedsludge collected from the sedimentation tank to the denitrification tankin the first stage.

[0017] 7. The system for treating organic wastewater according to anyone of 1 to 6 above, which further comprises an anaerobic tank connectedupstream of the denitrification tank in the first stage, and a pipe fordistributing feed water connected to the anaerobic tank.

[0018] 8. The system for treating organic wastewater according to anyone of 1 to 7 above, wherein the denitrification and nitrification tanksare at least partially filled with a carrier capable of holding livingbacterial cells.

[0019] 9. A method for treating organic wastewater in an organicwastewater treatment system comprising a multistage treatment tankhaving at least two stages in series, each stage comprising adenitrification tank and a nitrification tank connected in this order,said method comprising:

[0020] distributing feed water to the denitrification tank in eachstage;

[0021] filtering and separating at least a part of an activated sludgemixed liquor present in at least one nitrification tank, and;

[0022] supplying at least a part of the concentrated sludge mixed liquorobtained by said filtration and separation treatment to thedenitrification tank.

[0023] 10. The method according to 9 above, wherein the filtration andseparation treatment is accomplished with a filtration and separationmeans placed in the nitrification tank(s), and at least a part of theconcentrated sludge mixed liquor obtained by filtration and separationtreatment in the nitrification tank is supplied to the denitrificationtank.

[0024] 11. The method according to 9 above wherein the filtration andseparation treatment of the activated sludge mixed liquor isaccomplished by supplying at least a part of the activated sludge mixedliquor present in at least one nitrification tank to a solid-liquidseparation tank having a filtration and separation means placed therein,and at least a part of the concentrated sludge mixed liquor obtained byfiltration and separation treatment in the solid-liquid separation tankis supplied to the denitrification tank.

[0025] 12. The method according to any one of 9 to 11 above, wherein thefiltration and separation means is a dynamic filter comprising awater-permeable filtration layer support material for forming a dynamicfiltration layer of activated sludge particles thereon.

[0026] 13. The method according to 12 above, wherein the water-permeablefiltration layer support material is composed of at least one memberselected from the group consisting of a woven fabric material, anonwoven fabric material and a metallic net material.

[0027] 14. The method according to any one of 9 to 13 above, wherein atleast a part of the activated sludge mixed liquor present in thenitrification tank in the final stage is introduced into a sedimentationtank and subjected to solid-liquid separation, and at least a part ofthe deposited sludge collected from the sedimentation tank is returnedto the denitrification tank in the first stage.

[0028] 15. The method according to any one of 9 to 14 above,

[0029] wherein an anaerobic tank is further connected upstream of thedenitrification tank in the first stage, and feed water is alsodistributed to the anaerobic tank.

[0030] 16. The method according to any one of 9 to 15 above, wherein thedenitrification and nitrification tanks are at least partially filledwith a carrier capable of holding living bacterial cells.

[0031] According to the present invention, in treating organicwastewater in an organic wastewater treatment system comprising amultistage treatment tank having at least two stages in series, eachstage comprising a denitrification tank and a nitrification tankconnected in this order, the sludge concentration in the denitrificationtanks can be maintained at a high level by distributing feed water tothe denitrification tank in each stage and supplying an activated sludgemixed liquor in the nitrification tank to the denitrification tank afterbeing concentrated by filtration and separation. In addition to this, itis possible to achieve a significantly reduced level of NO_(x)—N intreated water and improved removal of T-N (total nitrogen) by supplyingNO_(x)—N in the nitrified fluid to the denitrification tank and reducingit to N₂ by the action of denitrifying bacteria present in thedenitrification tank. Moreover, since the concentrated sludge mixedliquor is obtained by filtration and separation of the sludge mixedliquor to remove treated water before being supplied to thedenitrification tank, the concentrated sludge mixed liquor supplied tothe denitrification tank has an extremely low level of dissolved oxygen.Even if this sludge mixed liquor is returned to the denitrificationtank, the risk of reduced denitrification performance due to dissolvedoxygen is very small.

[0032] A filtration and separation means available for use in filtrationand separation treatment may be any conventionally known immersion-typemembrane separator. In a preferred embodiment of the present invention,filtered water can be stably obtained at low hydraulic head pressure byusing what is called a “dynamic filter module” as a means for filteringand separating the sludge mixed liquor in the nitrification tank. Forexample, such a filtration and separation means may be immersed in thenitrification tank.

[0033] Alternatively, in another embodiment of the present invention,such a filtration and separation means as mentioned above may beimmersed in a solid-liquid separation tank provided separately. Thesolid-liquid separation tank may be equipped with a pipe for supplyingan activated sludge mixed liquor in the nitrification tank to thesolid-liquid separation tank. After filtration and separation of theactivated sludge mixed liquor in the solid-liquid separation tank, theresulting concentrated sludge mixed liquor may be supplied to thedenitrification tank.

[0034] In particular, when using a dynamic filter as a filtration andseparation means, dynamic filtration is preferably accomplished in asolid-liquid separation tank provided as stated above. In this case, anactivated sludge mixed liquor to be introduced is preferably kept asfree from aeration bubbles as possible. In contrast, when a dynamicfilter is immersed in the nitrification tank, it is necessary to form adownward cross-flow on the filter surface with the aid of spiral flowgenerated by aeration; and hence water flow on the filter surface may beconsiderably nonuniform because it is significantly affected by theposition and distance of an aeration pipe relative to the filter. Sincea nonuniform flow of the sludge mixed liquor on the filter surface makesit impossible to ensure a uniform thickness of the dynamic filtrationlayer formed on the surface, it is difficult to maintain a stable volumeof filtered water. Also, when a dynamic filter is placed in thenitrification tank (aeration tank), a dynamic filtration layer will beformed in the presence of aeration bubbles. However, under suchconditions, the formed dynamic filtration layer is easy to flake off bythe action of aeration bubbles, and it may be difficult to maintain thequality of filtered water. Because the water level in the nitrificationtank will vary depending on the volume of feed water and aeration, thewater head difference between the nitrification tank and its downstreamtank is also variable and hence results in unstable filtration pressure.This not only causes variations in the volume of filtered water, butalso adversely affects the filtration performance of the sludgefiltration layer formed on the filter surface. However, when a dynamicfilter is immersed in a solid-liquid separation tank provided separatelyfrom the multistage structure of denitrification and nitrificationtanks, and an activated sludge mixed liquor in the nitrification tank issupplied to the solid-liquid separation tank where the activated sludgemixed liquor is filtered and separated by dynamic filtration, treatedwater of good quality can be stably obtained with a constant volume offiltered water even if there are variations in the volume and quality ofraw feed water and changes in the properties of activated sludgeparticles. In such a case where a solid-liquid separation tank isprovided separately, the level of dissolved oxygen in the solid-liquidseparation tank is lower at a position closer to the outlet because thesolid-liquid separation tank receives only the sludge mixed liquorsupplied from the nitrification tank while discharging treated water.Namely, the concentrated sludge mixed liquor discharged from thesolid-liquid separation tank is almost free from dissolved oxygen andposes no risk of reduced denitrification performance due to dissolvedoxygen even if the mixed liquor is returned to the denitrification tank.

[0035] The filtration and separation treatment may be performed in thenitrification tank in each stage and the resulting concentratedactivated sludge mixed liquor may then be returned to the correspondingdenitrification tank in the same stage. This enables the supply of theconcentrated activated sludge mixed liquor to the denitrification tankin each stage, thereby making it possible to maintain a high level ofMLSS (mixed liquor suspended solids) in the denitrification andnitrification tanks and to ensure high removal of nitrogen. Moreover,the nitrified fluid from an upstream nitrification tank (e.g., tank 1Cin FIG. 1 explained below) is efficiently denitrified in its downstreamdenitrification tank (tank 2B in FIG. 1) with the aid of raw watersupplied thereto. Alternatively, in another embodiment of the presentinvention, the filtration and separation treatment may be performed insome of the nitrification tanks, in particular, exclusively in adownstream nitrification tank(s). For example, the filtration andseparation treatment may be performed exclusively on the activatedsludge mixed liquor in the nitrification tank in the final stage and theresulting concentrated sludge mixed liquor may then be returned to anyupstream denitrification tank.

[0036] In yet another embodiment of the present invention, asedimentation tank may be further provided downstream of thenitrification tank in the final stage of the multiple denitrificationand nitrification tanks connected in series. At least a part of theactivated sludge mixed liquor in the final stage may be introduced intothe sedimentation tank and subjected to solid-liquid separation. Such aconfiguration ensures final solid-liquid separation in the sedimentationtank even if variations in the volume and quality of raw water lead to areduction in the volume of filtered water obtained in each stage. Inaddition, it is possible to maintain a higher level of MLSS in thedenitrification tanks and to achieve higher denitrification performanceif at least a part of the deposited sludge obtained in the sedimentationtank is returned to the denitrification tank in the first stage.

[0037] The denitrification and nitrification tanks in each stage aredesigned to ensure that the filtered water obtained by filtration andseparation of the nitrified fluid from the nitrification tank can bedischarged as treated water from the system. Thus, they aresignificantly advantageous in maintaining a high level of MLSS in thetanks, in ensuring high removal of nitrogen, and in treating a largervolume of water.

[0038] A water-permeable filtration layer support material to be used inthe dynamic filter to form a dynamic filtration layer may be any of awoven fabric, a nonwoven fabric, a metallic net or the like, all ofwhich produce the same results. In the case of using a woven fabric or ametallic net, those having a pore size of 50 to 200 μm are suitable forthis purpose. The filter is primarily designed to have a planar shape,but may also be designed to have a cylindrical or hollow shape. Two ormore filters may be bundled together into a module filter.

[0039] In a case where raw water to be treated has a high phosphoruscontent and hence requires biological phosphorus removal, an anaerobictank may be newly provided upstream of the denitrification tank in thefirst stage, and return sludge may be supplied to the anaerobic tanktogether with a part of feed water. This allows selective growth ofpolyphosphate-accumulating organisms (PAO) in the anaerobic tank anduptake of phosphorus contained in raw water by PAO in the nitrificationtanks, thereby also achieving removal of phosphorus contained in rawwater. If a sedimentation tank is provided downstream of thenitrification tank in the final stage in this embodiment, the depositedsludge collected from the sedimentation tank may be returned to theanaerobic tank. Alternatively, if the sludge is not returned from thefinal stage of the treatment tank, at least a part of the sludge mixedliquor in the denitrification tank in the first stage may be returned tothe anaerobic tank.

[0040] Such an anaerobic tank provided for the purpose of removingphosphorus is intended to mean a reaction tank free from DO (dissolvedoxygen) and NO_(x)—N. The use of such a tank allows preferential growthof polyphosphate-accumulating organisms throughout the entire system forbiological treatment. As a result, phosphorus contained in raw water iscaptured as polyphosphate by polyphosphate-accumulating organisms.

[0041] Further, in the present invention, the denitrification andnitrification tanks in each stage are at least partially filled with acarrier capable of holding living bacterial cells. When filled into thedenitrification and/or nitrification tanks, such a carrier isadvantageous in reducing the retention time and the tank volume.Examples of a carrier available for this purpose include polymerparticles (particle size: 2 to 4 mm) or sponge materials made of PEG(polyethylene glycol), PVA (polyvinyl alcohol), etc.

BRIEF DESCRIPTION OF DRAWINGS

[0042]FIG. 1 is a conceptual illustration of a system for organicwastewater treatment according to one embodiment of the presentinvention.

[0043]FIG. 2 is a conceptual illustration of a system for organicwastewater treatment according to another embodiment of the presentinvention.

[0044]FIG. 3 is a conceptual illustration of a system for organicwastewater treatment according to yet another embodiment of the presentinvention.

[0045]FIG. 4 is a conceptual illustration of a system for organicwastewater treatment according to yet another embodiment of the presentinvention.

[0046]FIG. 5 is a graph showing the time course of filtration fluxaveraged over the solid-liquid separation tanks used in one example ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0047] Detailed explanations will be given for various possibleembodiments of the present invention by reference to the accompanyingdrawings.

[0048]FIG. 1 is a flow sheet showing an example of raw water treatmentusing a system for organic wastewater treatment according to oneembodiment of the present invention.

[0049] The system for organic wastewater treatment shown in FIG. 1comprises a treatment tank having multiple stages in series, each stagecomprising denitrification (B) and nitrification (C) tanks connected inthis order. In FIG. 1, three sets of denitrification and nitrificationtanks are connected in series and in fluid communication with each otherto give the multistage structure of 1B-1C-2B-2C-3B-3C. In this treatmentsystem, raw feed water 1 is divided into three lines 1A, 2A and 3A,which are distributed to a first denitrification tank 1B, a seconddenitrification tank 2B and a third denitrification tank 3B,respectively. This allows the use of organic matter in the raw water asa hydrogen donor for denitrification and facilitates denitrificationunder anaerobic conditions. The effluent from the first denitrificationtank 1B enters its downstream first nitrification tank 1C, where NH₄—N(ammonia nitrogen) is nitrified. Each nitrification tank is equippedwith an air diffuser 9 connected to an air supply pipe 8, through whichaeration is accomplished to maintain the tank under an aerobicatmosphere. The nitrified fluid (activated sludge mixed liquor) from thefirst nitrification tank 1C is supplied to a first solid-liquidseparation tank 1D and then filtered through a filter module placed inthe solid-liquid separation tank 1D to give first filtered water 1F. Itshould be noted that the volume of filtered water is adjusted to notmore than that of raw feed water. The concentrated sludge mixed liquorremaining after filtration is circulated as first circulating sludge 1Eand enters the first denitrification tank 1B, where NO_(x)—N isdenitrified. A part of the nitrified fluid from the first nitrificationtank 1C is also delivered to its downstream second denitrification tank2B.

[0050] The above also applies to the second denitrification andnitrification tanks 2B and 2C. The sludge mixed liquor in thenitrification tank is supplied to a second solid-liquid separation tank2D to give filtered water 2F. The concentrated sludge mixed liquorremaining after filtration is circulated as circulating sludge 2E andenters the second denitrification tank 2B, where NO_(x)—N in thenitrified fluid is denitrified. This also applies to the thirddenitrification and nitrification tanks 3B and 3C. The nitrified fluidis supplied to a solid-liquid separation tank 3D and subjected tofiltration. The resulting concentrated sludge mixed liquor is circulatedas circulating sludge 3E and enters the third denitrification tank 3B.

[0051] It should be noted that the volume of filtered water obtainedfrom the first to third solid-liquid separation tanks is adjusted to notmore than that of raw feed water. In the system shown in FIG. 1.effluent 4 from the third nitrification tank 3C (final stage in thesystem shown in FIG. 1) enters the downstream sedimentation tank 5.After deposition and concentration, the resulting supernatant isobtained as treated water 6. The deposited sludge (concentrated sludge)collected from sedimentation tank 5 is returned to the firstdenitrification tank 1B as return sludge 7.

[0052] The filtration and separation means to be placed in thesolid-liquid separation tank may be any immersion-type membraneseparator known in the art. Alternatively, what is called a “dynamicfilter module” may also be used for this purpose. The use of a dynamicfilter module enables a stable provision of filtered water at lowhydraulic head pressure. In a case where such a dynamic filter module isplaced in the solid-liquid separation tank, there is no particular needto use a means for generating a cross-flow of the sludge mixed liquoralong the surface of a filtration layer support material in thesolid-liquid separation tank. That is, the flow rate of circulatingsludge through the separation tank is sufficient to form a dynamicfiltration layer. When the dynamic filter module is placed in thesolid-liquid separation tank, it is preferable to equip the solid-liquidseparation tank together with an air-diffusing pipe for air washing thedynamic filter.

[0053] Although FIG. 1 illustrates a configuration where thesedimentation tank 5 is placed downstream of the final nitrificationtank, it is also possible to not place the sedimentation tank 5downstream of the final nitrification tank. In this case, the filteredwater obtained from the final solid-liquid separation tank is collectedas treated water. It should be noted that when the sedimentation tank 5is not placed, the total volume of filtered water from each solid-liquidseparation tank is adjusted to the same level as that of raw feed water.

[0054] Filtration operation may be conducted in one or two of thesolid-liquid separation tanks depending on the volume and quality of rawfeed water and the quality of treated water; or, the circulating sludge3E from the third solid-liquid separation tank 3D may be circulated tothe first or second denitrification tank 1B or 2B. If higher removal ofnitrogen is required, the circulating sludge from the third solid-liquidseparation tank. 3D is desirably circulated to the first denitrificationtank 1B.

[0055] To illustrate another embodiment of the present invention, FIG. 2shows a configuration example of a system for organic wastewatertreatment wherein filtration and separation means is immersed in thenitrification tank. In the system for organic wastewater treatment shownin FIG. 2, filtration and separation means 1G, 2G and 3G are immersed ina first nitrification tank 1C, a second nitrification tank 2C and athird nitrification tank 3C, respectively. All other elements are thesame as shown in FIG. 1. Filtered water (1F, 2F, 3F) is obtained by thefiltration and separation means immersed in each nitrification tank. Inaddition, the concentrated sludge mixed liquor remaining afterfiltration is returned from each nitrification tank to the correspondingdenitrification tank in the same stage (1H, 2H, 3H). The filtration andseparation means to be immersed in each nitrification tank may be anyconventionally known immersion-type membrane separator. Alternatively,what is called a “dynamic filter module” may also be used for thispurpose. In a case where such a dynamic filter module is used, themodule is preferably placed apart from an air diffuser 9 to be placed ineach nitrification tank, such that a downward cross-flow is formed alongthe surface of the dynamic filter module with the aid of aerationgenerated by the air diffuser. Such a configuration allows stableformation of a dynamic filtration layer without the risk of flaking offthe formed dynamic filtration layer by aeration.

[0056] Moreover, the filtered water obtained through the dynamic filtermodule may also be supplied to an additional sedimentation tank and, inturn, the resulting supernatant may be collected as treated water whenthe dynamic filter module is used as a filtration and separation means.

[0057] In a case where raw feed water has a high phosphorus content andfurther requires biological phosphorous removal treatment, an anaerobictank may be newly provided upstream of the first denitrification tank,and return sludge may be supplied to the anaerobic tank together with apart of raw feed water. Such a configuration allows removal ofphosphorus contained in raw water. FIGS. 3 and 4 each show aconfiguration example of a system which further comprises an anaerobictank newly provided upstream of the first denitrification tank.

[0058] The system shown in FIG. 3 has the same configuration as shown inFIG. 1, except that an anaerobic tank 10 is newly provided upstream ofthe first denitrification tank 1B. In this case, the return sludge fromthe final sedimentation tank 5 is supplied to the anaerobic tank 10. Rawwater 1 is also distributed to the anaerobic tank 10 through a branchpipe 10A. In the system shown in FIG. 3, it should be noted that rawwater 1 is not distributed to the first denitrification tank 1B.Likewise, the system shown in FIG. 4 has the same configuration as shownin FIG. 2, except that an anaerobic tank 10 is newly provided upstreamof the first denitrification tank 1B.

[0059] The present invention will be further described in the followingexamples, which are not intended to limit the scope of the invention.

EXAMPLE 1

[0060] Housing complex sewage was treated using the organic wastewatertreatment system shown in FIG. 1.

[0061] Table 1 shows the treatment conditions used in this example.

[0062] Raw water was distributed at 5 m³/d to the first to thirddenitrification tanks 1B, 2B and 3B, respectively. The volume ofcirculating sludge supplied from each solid-liquid separation tank tothe corresponding denitrification tank was set to 15 m³/d. The volume offiltered water obtained from the first to third solid-liquid separationtanks 1D, 2D and 3D was set to 4 m³/d, 4.5 m³/d and 5 m³/d,respectively. Return sludge was returned at 2 m³/d from thesedimentation tank 5 to the first denitrification tank 1B. MLSS in thefirst denitrification and nitrification tanks was 4000 mg/L. MLSS in thesecond and subsequent denitrification and nitrification tanks was 3500mg/L. TABLE 1 Treatment conditions for denitrification and nitrificationtanks in each stage First Second denitrification denitrification Thirdand and denitrification and nitrification nitrification nitrificationtanks tanks tanks Introduced raw feed 5 5 5 water (m³/d) Circulating 1515 15 sludge (m³/d) Filtered 4 4.5 5.0 water (m³/d) Return 2 — — sludge(m³/d) MLSS (mg/L) 4000 3500 3500

[0063] In this example, three sheets of a water-permeable planar filter(effective filtration area: 0.6 m²/sheet) were used as a filter moduleand immersed in each of the first to third solid-liquid separationtanks. The water-permeable filter used for this purpose was a polyesterwoven fabric having a thickness of about 0.1 mm and a pore size of 114μm. The hydraulic head pressure during filtration was set to about 10cm, while a cross-flow of the sludge mixed liquor on the filter surfacewas adjusted to have an average flow rate of 0.015 m/s. Under theseconditions, a dynamic filtration layer of sludge was formed on thefilter surface and provided for filtration.

[0064] Filtration operation was stopped every 2 hours for washing thedynamic filter. First, the outside of the filter was washed with airbubbles (air wash) by aeration through an air-diffusing pipe providedbelow the filter. Then, water was introduced into the inside of thefilter for washing the inside of the filter (water backwash). Regularoperation for filtration was started after water backwash, provided thatduring a period required for sludge discharge the resulting filteredwater was not collected and returned as sludge to the denitrificationtanks.

[0065] Table 2 shows treatment conditions for the solid-liquidseparation tanks. TABLE 2 Treatment conditions for eachfiltration/separation tank Supplied sludge (m³/d) 19 Filtered water(m³/d) 4.0 to 5.0 Air wash volume (Nm³/m²/d)* 2.5 Air wash time (min) 3Water backwash volume (m³/m²/d)** 40 Water backwash time (min) 0.5Sludge discharge time (min) 3 Filtration/washing interval Once every 2.0h

[0066]FIG. 5 shows the time course of filtration flux averaged over thefirst to third solid-liquid separation tanks in this example.

[0067] During about 2 months after starting the treatment, thefiltration flux was almost maintained at 2.7 m/d, indicating that thesystem ensured stable treatment.

[0068] In addition, Table 3 shows the average quality of raw water andtotal treated water. As used herein, the “average quality of totaltreated water” is intended to mean the quality averaged over thefiltered water obtained from all the solid-liquid separation tanks andthe effluent obtained from the sedimentation tank.

[0069] SS in the raw feed water was 120 mg/L, whereas SS in the totaltreated water was 5.6 mg/L.

[0070] On the other hand, NH₄—N in the raw feed water was 45 mg/L,whereas NH₄—N in the total treated water was reduced to 0.5 mg/L afternitrification/denitrification treatment, indicating that the systemensured almost complete nitrification. NO_(x)—N in the total treatedwater was 5.0 mg/L. T-N was 62 mg/L in the raw water, whereas it wasreduced to 7.5 mg/L in the total treated water, indicating that thesystem achieved about 88% removal of nitrogen. TABLE 3 Quality of rawwater and total treated water Raw water Total treated water pH 7.1 7.3SS (mg/L) 120 5.6 BOD (mg/L) 230 ≦5 NH₄—N (mg/L) 45 0.5 NO_(x)—N (mg/L)ND 5.0 T-N (mg/L) 62 7.5 T-P (mg/L) 5.0 3.5

EXAMPLE 2

[0071] Housing complex sewage was treated using the organic wastewatertreatment system shown in FIG. 3.

[0072] Table 4 shows the treatment conditions used in this example.

[0073] Raw water was distributed at 5 m³/d to the anaerobic tank 10 aswell as the second and third denitrification tanks 2B and 3B,respectively. The volume of circulating sludge supplied from eachsolid-liquid separation tank to the corresponding denitrification tankwas set to 15 m³/d. The volume of filtered water obtained from the firstto third solid-liquid separation tanks 1D, 2D and 3D was set to 4 m³/d,4.5 m³/d and 5 m³/d, respectively. Return sludge was returned at 2 m³/dfrom the sedimentation tank 5 to the anaerobic tank 10. MLSS of themixed liquor in the anaerobic tank 10 was 4000 mg/L. MLSS in the secondand subsequent denitrification and nitrification tanks was 3500 mg/L.TABLE 4 Treatment conditions for anaerobic tank and denitrification andnitrification tanks First Second denitri- denitri- Third ficationfication denitrification and and and anaerobic nitrificationnitrification nitrification tank tanks tanks tanks Introduced 5 — 5 5raw feed water (m³/d) Circulating — 15 15 15 sludge (m³/d) Filtered — 44.5 5.0 water (m³/d) Return 2 — — — sludge (m³/d) MLSS (mg/L) 4000 40003500 3500

[0074] In this example, three sheets of a water-permeable planar filter(effective filtration area: 0.6 m²/sheet) were used as a filter moduleand immersed in each of the first to third solid-liquid separationtanks. The water-permeable filter used for this purpose was a polyesterwoven fabric having a thickness of about 0.1 mm and a pore size of 114μm. The hydraulic head pressure during filtration was set to about 10cm, while a cross-flow of the sludge mixed liquor on the filter surfacewas adjusted to have an average flow rate of 0.015 m/s. Under theseconditions, a dynamic filtration layer of sludge was formed on thefilter surface and provided for filtration.

[0075] Filtration operation was stopped every 2 hours for washing thedynamic filter. First, the outside of the filter was washed with airbubbles (air wash) by aeration through an air-diffusing pipe providedbelow the filter. Then, water was introduced into the inside of thefilter for washing the inside of the filter (water backwash). Regularoperation for filtration was started after water backwash, provided thatduring a period required for sludge discharge the resulting filteredwater was not collected and returned as sludge to the denitrificationtanks.

[0076] Table 5 shows treatment conditions for the solid-liquidseparation tanks. TABLE 5 Treatment conditions for eachfiltration/separation tank Supplied sludge (m³/d) 19 Filtered water(m³/d) 4.0 to 5.0 Air wash volume (Nm³/m²/d)* 2.5 Air wash time (min) 3Water backwash volume (m³/m²/d)** 40 Water backwash time (min) 0.5Sludge discharge time (min) 3 Filtration/washing interval Once every 2.0h

[0077] During about 2 months after starting the treatment, thefiltration flux was almost maintained at 2.7 m/d, indicating that thesystem ensured stable treatment.

[0078] In addition, Table 6 shows the average quality of raw water andtotal treated water. As used herein, the “average quality of totaltreated water” is intended to mean the quality averaged over thefiltered water obtained from all the solid-liquid separation tanks andthe effluent obtained from the sedimentation tank.

[0079] SS in the raw feed water was 120 mg/L, whereas SS in the totaltreated water was 6.5 mg/L. As a result of using the anaerobic tank 10,which allowed growth of polyphosphate-accumulating organisms, T-P (totalphosphorus) was reduced to 0.8 mg/L in the total treated water ascompared to 4 mg/L in the raw water. Also, the anaerobic tank 10 wasfound to have a phosphorus (PO₄—P) content of 25 mg/L due to thepresence of polyphosphate-accumulating organisms.

[0080] On the other hand, NH₄—N in the raw feed water was 45 mg/L,whereas NH₄—N in the total treated water was reduced to 0.5 mg/L afternitrification/denitrification treatment, indicating that the systemensured almost complete nitrification. NO_(x)—N in the total treatedwater was 5.0 mg/L. T-N was 62 mg/L in the raw water, whereas it wasreduced to 7.5 mg/L in the total treated water, indicating that thesystem achieved about 88% removal of nitrogen. TABLE 6 Quality of rawwater and total treated water Raw water Total treated water pH 7.1 7.3SS (mg/L) 120 6.5 BOD (mg/L) 230 ≦5 NH₄—N (mg/L) 45 0.5 NO_(x)—N (mg/L)ND 5.0 T-N (mg/L) 62 7.5 T-P (mg/L) 5.0 3.5 T-P (mg/L) 4 0.8

INDUSTRIAL APPLICABILITY

[0081] According to the present invention, in a system comprising amultistage treatment tank having at least two stages in series, eachstage comprising a denitrification tank and a nitrification tankconnected in this order, the sludge concentration in the denitrificationtanks can be maintained at a high level by distributing feed water tothe denitrification tank in each stage and returning an activated sludgemixed liquor in the nitrification tank to the denitrification tank afterbeing concentrated by filtration and separation. In addition, it ispossible to achieve a significantly reduced level of NO_(x)—N in treatedwater and improved removal of T-N by supplying NO_(x)—N in the nitrifiedfluid to the denitrification tank and reducing it to N₂ by the action ofdenitrifying bacteria present in the denitrification tank.

[0082] In addition, since the concentrated sludge mixed liquor isobtained by filtration and separation of the filtered sludge mixedliquor to remove treated water before being supplied to thedenitrification tank, the concentrated sludge mixed liquor supplied tothe denitrification tank has an extremely low level of dissolved oxygen.Even if this sludge mixed liquor is returned to the denitrificationtank, the risk of reduced denitrification performance due to dissolvedoxygen is very small.

[0083] Further, in one embodiment of the present invention where anactivated sludge mixed liquor in the nitrification tank is supplied to asolid-liquid separation tank having a filtration means immersed therein,treated water of good quality can be stably obtained with a constantvolume of filtered water even if there are variations in the volume andquality of raw feed water and changes in the properties of activatedsludge particles.

[0084] Furthermore, in a preferred embodiment of the present inventionwhere a sedimentation tank is further provided downstream of the finalnitrification tank, such a configuration ensures solid-liquid separationin the sedimentation tank even if variations in the volume and qualityof raw water lead to a reduction in the volume of filtered water.Moreover, since the major part of treated water is discharged asfiltered water with the aid of each filtration means, a small volume offeed water enters the sedimentation tank and results in a highconcentration of return sludge. It is therefore possible to maintain ahigher level of MLSS in the denitrification tanks and to achieve higherdenitrification performance if such highly concentrated sludge isreturned to the denitrification tanks.

[0085] The denitrification and nitrification tanks in each stage aredesigned to ensure that the sludge mixed liquor in the nitrificationtank is filtered through the filtration means to discharge filteredwater from the system; hence they are significantly advantageous inmaintaining a high level of MLSS in the tanks, in ensuring high removalof nitrogen, and in treating a larger volume of water.

1-16. (Cancelled).
 17. A system for treating organic wastewater, whichcomprises a multistage treatment tank having at least two stages inseries, each stage comprising a denitrification tank and a nitrificationtank connected in this order, wherein said system comprises a pipe fordistributing feed water to the denitrification tank in each stage, ameans for filtering and separating at least a part of an activatedsludge mixed liquor in at least one nitrification tank, and a pipe forsupplying at least a part of the concentrated sludge mixed liquorobtained by said filtration and separation treatment to thedenitrification tank.
 18. The system for treating organic wastewateraccording to claim 17, wherein the filtration and separation means isplaced in the nitrification tank, and at least a part of theconcentrated sludge mixed liquor obtained by filtration and separationtreatment in the nitrification tank is supplied to the denitrificationtank.
 19. The system for treating organic wastewater according to claim17, which further comprises a solid-liquid separation tank, wherein thefiltration and separation means is placed in the solid-liquid separationtank, and at least a part of the concentrated sludge mixed liquorobtained by filtration and separation treatment in the solid-liquidseparation tank is supplied to the denitrification tank.
 20. The systemfor treating organic wastewater according to claim 17, wherein thefiltration and separation means is a dynamic filter comprising awater-permeable filtration layer support material for forming a dynamicfiltration layer thereon.
 21. The system for treating organic wastewateraccording to claim 20, wherein the water-permeable filtration layersupport material is composed of at least one member selected from thegroup consisting of a woven fabric material, a nonwoven fabric materialand a metallic net material.
 22. The system for treating organicwastewater according to claim 17, which further comprises asedimentation tank to be used for solid-liquid separation of at least apart of the activated sludge mixed liquor introduced from thenitrification tank in the final stage, as well as a pipe for returningthe deposited sludge collected from the sedimentation tank to thedenitrification tank in the first stage.
 23. The system for treatingorganic wastewater according to claim 17, which further comprises ananaerobic tank connected upstream of the denitrification tank in thefirst stage, and a pipe for distributing feed water connected to theanaerobic tank.
 24. The system for treating organic wastewater accordingto claim 17, wherein the denitrification and nitrification tanks are atleast partially filled with a carrier capable of holding livingbacterial cells.
 25. A method for treating organic wastewater in anorganic wastewater treatment system comprising a multistage treatmenttank having at least two stages in series, each stage comprising adenitrification tank and a nitrification tank connected in this order,said method comprising: distributing feed water to the denitrificationtank in each stage; filtering and separating at least a part of anactivated sludge mixed liquor in at least one nitrification tank, and;supplying at least a part of the concentrated sludge mixed liquorobtained by said filtration and separation treatment to thedenitrification tank.
 26. The method according to claim 9, wherein thefiltration and separation treatment is accomplished with a filtrationand separation means placed in the nitrification tank, and at least apart of the concentrated sludge mixed liquor obtained by filtration andseparation treatment in the nitrification tank is supplied to thedenitrification tank.
 27. The method according to claim 25, wherein thefiltration and separation treatment of the activated sludge mixed liquoris accomplished by supplying at least a part of the activated sludgemixed liquor in at least one nitrification tank to a solid-liquidseparation tank having a filtration and separation means placed therein,and at least a part of the concentrated sludge mixed liquor obtained byfiltration and separation treatment in the solid-liquid separation tankis supplied to the denitrification tank.
 28. The method according toclaim 25, wherein the filtration and separation means is a dynamicfilter comprising a water-permeable filtration layer support materialfor forming a dynamic filtration layer thereon.
 29. The method accordingto claim 28, wherein the water-permeable filtration layer supportmaterial is composed of at least one member selected from the groupconsisting of a woven fabric material, a nonwoven fabric material and ametallic net material.
 30. The method according to claim 25, wherein atleast a part of the activated sludge mixed liquor in the nitrificationtank in the final stage is introduced into a sedimentation tank andsubjected to solid-liquid separation, and the deposited sludge collectedfrom the sedimentation tank is returned to the denitrification tank inthe first stage.
 31. The method according to claim 25, wherein ananaerobic tank is further connected upstream of the denitrification tankin the first stage, and feed water is also distributed to the anaerobictank.
 32. The method according to claim 25, wherein the denitrificationand nitrification tanks are at least partially filled with a carriercapable of holding living bacterial cells.